The present invention relates to sensors for magnetic clamping systems and, in particular, a sensing system for assuring proper magnetization and continual material contact for magnetic quick mold change systems on injection molding machines.
Material handling and manufacturing systems, such as injection molding machines, are generally provided with various sensing systems for assuring that coupling mechanisms are properly engaged for operation thereof to continue. Such systems for mechanically and hydraulically based systems are well established. Recently, magnetic coupling systems have been increasingly used in quick mold change systems for injection molding machines. Magnetic coupling systems have provided a vehicle for minimizing down time and maximizing productivity. The magnetic based quick mold change systems, unlike mechanical and hydraulic systems, do not require extensive modifications to the mold and lend themselves to magnetic clamping with minimal structural modification inasmuch as they may be readily directly coupled with the flat steel bases customarily used for injection molds. Additionally, magnetic clamps have greater clamping accuracy, improved safety, ready accessibility, and low maintenance and energy consumption.
The quick mold change magnetic systems do not use redundant coupling systems, relying solely on the magnetization for maintaining the mold in accurate and functional position. The quoted clamping force for these systems is generally based on a mild steel back-plate of adequate thickness and good surface finish. Under normal operating conditions, the magnetic clamping forces are ample to hold the mold on the mold platen. Certain conditions, however, can arise wherein large forces may transiently occur that have the potential to dislodge the mold. Over-packing the mold or incorrect ejector stroke adjustment, for example, interrupt the magnetic circuits sufficiently for the mold to be released and fall to the bed of the machine. Further, the mold may become warped or covered in rust or dirt, or a foreign body may get trapped between the mold and the chuck. All of the foregoing problems can reduce the applied clamping force of the magnet system. When such conditions occur no adequate provisions are made for detection. Current sensors merely detect problems with electrical supply. Existing proximity probes detect the physical presence of the mold. Neither fully takes into account the magnetic performance of the poles in contact with the mold.
The proximity sensors have been incorporated into the magnetic coupler. In operation, as long as the mold is in contact with the magnetic chuck, the proximity sensors are closed, and operating conditions maintained. In the event the mold separates a predetermined distance from the magnetic chuck, generally 0.2 mm or more, the proximity sensors open and an emergency stop condition is initiated at the machine. Such sensing parameters are not altogether adequate inasmuch as this threshold distance reduces the clamping force significantly and the mold will have already dislodged from its base prior to sensor signal.
A further problem can be introduced by the initial mounting of the mold. Therein, the proximity sensors prevent magnetization unless the mold is positioned within the threshold limit. This preassembly condition is difficult to attain in normal operations. Although not recommended, operators oftentimes increase the proximity switch threshold to permit operation of magnetization circuit. This approach, however, can result in insufficient magnetic coupling for sustained operation. The excessive spacing at time of magnetization prevents the switchable magnets from achieving full saturation, thereby reducing clamping force. Further, the threshold adjustment also increases the amount of tolerable mold movement before establishing the emergency stop condition, further increasing the possibility of mold separation occurring before the emergency signal.
The present invention provides a sensing system providing greater sensitivity to mold separation, reduction in mold mounting alignment conditions, and assurance of magnetic saturation in the coupled condition prior to operation. The sensor system determines acceptable magnetization between a magnetic clamp and a mold patent for a quick mold change system on an injection molding machine. The sensor system includes secondary sensing coils magnetically coupled with a switchable magnet in the clamp for effecting a voltage output in accordance with magnetic flux conditions between the magnets and the mold and providing a control signal based on changes in said flux condition for controlling activity of the workpiece in accordance therewith.
Accordingly, it is an object of the present invention to provide a magnetic coupling system that insures adequate clamping forces for continued operation.
Another object is to provide a sensor system for magnetic clamping devices that detects permeability conditions at the magnetic interface for determining acceptable clamping forces.
A further object of the invention is to provide a magnetic clamping system for quick mold change molding machines that detects deleterious mold movements in order to minimize the risk of the mold falling from the magnetic clamp.
Yet another object of the invention is to provide a sensor system for a clamping device that conditions continued operations based on flux-based movement and permeability detection.